Atheists often don't mind "admitting" to what the Bible says - they simply reject the idea that what it says is actually true. As in rejecting the virgin birth, the bodily ascension of Christ, the miracles of the bible and in this example they freely admit to what the Bible says - while rejecting it as 'truth'.
Professor James Barr, Regius Professor of Hebrew at the University of Oxford, has written:
‘Probably, so far as I know, there is no professor of Hebrew or Old Testament at any world-class university
who does not believe that the writer(s) of
Genesis 1–11 intended to convey to their readers the ideas that:
(a) creation took place in a series of six days which were the same as the days of 24 hours we now experience
(b) the figures contained in the Genesis genealogies provided by simple addition a chronology from the beginning of the world up to later stages in the biblical story
(c) Noah’s flood was understood to be world-wide and extinguish all human and animal life except for those in the ark.
Or, to put it negatively, the apologetic arguments which suppose the "days" of creation to be long eras of time, the figures of years not to be chronological, and the flood to be a merely local Mesopotamian flood, are not taken seriously by any such professors, as far as I know.’